This invention relates generally to the automatic steering of vehicles by scanning of a directrix or external course-determining facility. This directrix may be of the physical type, such as a furrow in the ground, a cutting edge of grain or other crops to be harvested, or a curbstone edge. The invention is also applicable to use with a directrix formed, for example, by a laser beam or the electro-magnetic field of a cable embedded in the ground. An appropriate sensor must accordingly be provided capable of scanning the associated directrix. In all cases, the driver of the vehicle is relieved of the tiring steering activity as long as a useable directrix is available thereby enabling the driver to concentrate attention on supervising operation of the engine, for example, or the cutting and threshing action of a combine. The driver is thereby able to make full use of all vehicle capabilities, such as speed of travel and width of cutting of a combine.
In connection with power assisted steering systems that are operated under automatic control, facilities must be provided for restoring the steering system to manual control operation. According to prior art arrangements as disclosed for example in a technical article by Hesse and Moeller appearing in "Landbauforschung Voelkenrade", issue No. 17 of 1973, pages 41-46, a sensor and regulating system for automatic steering operation is provided wherein both a feeler as well as a steering angle signal generator attached to a steerable wheel, deliver electric signals which operate an otherwise hydraulic regulating and control system. In an emergency situation, such as interruption in the operation of the directrix, vehicle steering must be restored to manual control. This was accomplished in prior art arrangements by means of an electrically operated restoring valve so that change-over from automatic to manual control required a special switching action before manual control through the steering control wheel became operative. Further, such prior art arrangements were highly susceptible to breakdown in the electrical control sections of the system to make mode switching unreliable.